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 * Copyright (c) 2005, 2006, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package javax.lang.model.util;

import static javax.lang.model.SourceVersion.RELEASE_6;

import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.element.PackageElement;
import javax.lang.model.element.TypeElement;
import javax.lang.model.element.TypeParameterElement;
import javax.lang.model.element.VariableElement;

/**
 * A scanning visitor of program elements with default behavior appropriate for
 * the {@link SourceVersion#RELEASE_6 RELEASE_6} source version. The
 * <tt>visit<i>XYZ</i></tt> methods in this class scan their component elements
 * by calling {@code scan} on their {@linkplain Element#getEnclosedElements
 * enclosed elements}, {@linkplain ExecutableElement#getParameters parameters},
 * etc., as indicated in the individual method specifications. A subclass can
 * control the order elements are visited by overriding the
 * <tt>visit<i>XYZ</i></tt> methods. Note that clients of a scanner may get the
 * desired behavior be invoking {@code v.scan(e, p)} rather than
 * {@code v.visit(e, p)} on the root objects of interest.
 * 
 * <p>
 * When a subclass overrides a <tt>visit<i>XYZ</i></tt> method, the new method
 * can cause the enclosed elements to be scanned in the default way by calling
 * <tt>super.visit<i>XYZ</i></tt>. In this fashion, the concrete visitor can
 * control the ordering of traversal over the component elements with respect to
 * the additional processing; for example, consistently calling
 * <tt>super.visit<i>XYZ</i></tt> at the start of the overridden methods will
 * yield a preorder traversal, etc. If the component elements should be
 * traversed in some other order, instead of calling
 * <tt>super.visit<i>XYZ</i></tt>, an overriding visit method should call
 * {@code scan} with the elements in the desired order.
 * 
 * <p>
 * Methods in this class may be overridden subject to their general contract.
 * Note that annotating methods in concrete subclasses with
 * {@link java.lang.Override @Override} will help ensure that methods are
 * overridden as intended.
 * 
 * <p>
 * <b>WARNING:</b> The {@code ElementVisitor} interface implemented by this
 * class may have methods added to it in the future to accommodate new,
 * currently unknown, language structures added to future versions of the
 * Java&trade; programming language. Therefore, methods whose names begin with
 * {@code "visit"} may be added to this class in the future; to avoid
 * incompatibilities, classes which extend this class should not declare any
 * instance methods with names beginning with {@code "visit"}.
 * 
 * <p>
 * When such a new visit method is added, the default implementation in this
 * class will be to call the {@link #visitUnknown visitUnknown} method. A new
 * element scanner visitor class will also be introduced to correspond to the
 * new language level; this visitor will have different default behavior for the
 * visit method in question. When the new visitor is introduced, all or portions
 * of this visitor may be deprecated.
 * 
 * @param <R>
 *            the return type of this visitor's methods. Use {@link Void} for
 *            visitors that do not need to return results.
 * @param <P>
 *            the type of the additional parameter to this visitor's methods.
 *            Use {@code Void} for visitors that do not need an additional
 *            parameter.
 * 
 * @author Joseph D. Darcy
 * @author Scott Seligman
 * @author Peter von der Ah&eacute;
 * @since 1.6
 */
@SupportedSourceVersion(RELEASE_6)
public class ElementScanner6<R, P> extends AbstractElementVisitor6<R, P> {
	/**
	 * The specified default value.
	 */
	protected final R DEFAULT_VALUE;

	/**
	 * Constructor for concrete subclasses; uses {@code null} for the default
	 * value.
	 */
	protected ElementScanner6() {
		DEFAULT_VALUE = null;
	}

	/**
	 * Constructor for concrete subclasses; uses the argument for the default
	 * value.
	 */
	protected ElementScanner6(R defaultValue) {
		DEFAULT_VALUE = defaultValue;
	}

	/**
	 * Iterates over the given elements and calls {@link #scan(Element, Object)
	 * scan(Element, P)} on each one. Returns the result of the last call to
	 * {@code scan} or {@code DEFAULT_VALUE} for an empty iterable.
	 * 
	 * @param iterable
	 *            the elements to scan
	 * @param p
	 *            additional parameter
	 * @return the scan of the last element or {@code DEFAULT_VALUE} if no
	 *         elements
	 */
	public final R scan(Iterable<? extends Element> iterable, P p) {
		R result = DEFAULT_VALUE;
		for (Element e : iterable)
			result = scan(e, p);
		return result;
	}

	/**
	 * Processes an element by calling {@code e.accept(this, p)}; this method
	 * may be overridden by subclasses.
	 * 
	 * @return the result of visiting {@code e}.
	 */
	public R scan(Element e, P p) {
		return e.accept(this, p);
	}

	/**
	 * Convenience method equivalent to {@code v.scan(e, null)}.
	 * 
	 * @return the result of scanning {@code e}.
	 */
	public final R scan(Element e) {
		return scan(e, null);
	}

	/**
	 * {@inheritDoc} This implementation scans the enclosed elements.
	 * 
	 * @param e
	 *            the element to visit
	 * @param p
	 *            a visitor-specified parameter
	 * @return the result of scanning
	 */
	public R visitPackage(PackageElement e, P p) {
		return scan(e.getEnclosedElements(), p);
	}

	/**
	 * {@inheritDoc} This implementation scans the enclosed elements.
	 * 
	 * @param e
	 *            the element to visit
	 * @param p
	 *            a visitor-specified parameter
	 * @return the result of scanning
	 */
	public R visitType(TypeElement e, P p) {
		return scan(e.getEnclosedElements(), p);
	}

	/**
	 * {@inheritDoc} This implementation scans the enclosed elements.
	 * 
	 * @param e
	 *            the element to visit
	 * @param p
	 *            a visitor-specified parameter
	 * @return the result of scanning
	 */
	public R visitVariable(VariableElement e, P p) {
		return scan(e.getEnclosedElements(), p);
	}

	/**
	 * {@inheritDoc} This implementation scans the parameters.
	 * 
	 * @param e
	 *            the element to visit
	 * @param p
	 *            a visitor-specified parameter
	 * @return the result of scanning
	 */
	public R visitExecutable(ExecutableElement e, P p) {
		return scan(e.getParameters(), p);
	}

	/**
	 * {@inheritDoc} This implementation scans the enclosed elements.
	 * 
	 * @param e
	 *            the element to visit
	 * @param p
	 *            a visitor-specified parameter
	 * @return the result of scanning
	 */
	public R visitTypeParameter(TypeParameterElement e, P p) {
		return scan(e.getEnclosedElements(), p);
	}
}
